Surface geometries have the ability to indicate whether they are single-sided or double-sided. The front and back sides of a surface may independently "exist" or "not exist." When surfaces indicate sidedness properties, they are typically double-sided, front-sided, or back-sided.

This ability to have "sides" is also true for geometry collections that may contain Surfaces, such as CompositeSurfaces, MultiSurfaces and Aggregates. If the sidedness of a container geometry does not match the sidedness of a surface within that container, the value on the surface should be used. That is, sidedness is not inherited from the container a surface is in.

The surfaces described in this documentation indicate whether they support sidedness properties.